Polytetrafluoroethylene (hereinafter referred to as PTFE) has excellent chemical resistance and heat resistance and thus is widely used as material for various industrial components.
In an application to e.g. slide members or sealing members among industrial components, PTFE itself is inadequate in the abrasion resistance or compression property, and in many cases, PTFE is used as mixed with a filler.
As the filler to improve the abrasion resistance or compression property, various fillers may be employed such as a glass fiber powder, a carbon fiber powder or a bronze powder. A filler-containing PTFE powder obtainable by mixing such a filler, is compression-molded, followed by cutting work to form a bearing, a gasket or the like.
However, the filler-containing PTFE powder has no substantial flowability and thus is difficult to use. Accordingly, it is usually subjected to a granulation process to form filler-containing PTFE granules having flowability imparted.
With such filler-containing PTFE granules, a phenomenon has been observed wherein the mixed filler falls off from the surface layer of the granules due to vibration during the transportation or other causes. Such a phenomenon is called detachment of the filler. If detachment of the filler is remarkable, the uniformity of the filler-containing PTFE granules deteriorates, which may adversely affect the appearance and physical properties of a molded product obtainable from such filler-containing PTFE granules. Further, there has been a problem that the flowability of the filler-containing PTFE granules tends to deteriorate.
In order to solve such problems, a method has been proposed wherein a small amount of a PTFE emulsion is added during the production of the filler-containing PTFE granules by an underwater agitation granulation method (Patent Document 1).
However, by the proposed method, a new problem has resulted such that upon completion of the granulation step, the PTFE emulsion not attached to the surface of the filler-containing PTFE granules is discharged together with the waste liquid, whereby the waste liquid becomes turbid. Further, the PTFE emulsion discharged without being attached to the filler-containing PTFE granules, is present in the waste liquid, and the mass of the discharged PTFE emulsion is hardly measurable, whereby there has been a problem that the PTFE content in the obtainable filler-containing PTFE granules can hardly be accurately grasped.
Patent Document 1: U.S. Pat. No. 3,980,612